This section provides background information related to the present disclosure which is not necessarily prior art.
In a typical datacenter environment, an uninterruptible power supply system (UPS system) provides emergency power to a load when the main power source fails protecting hardware such as computers, servers or other electrical equipment that could cause serious business disruptions or data loss. It provides near-instantaneous protection from power interruptions by supplying energy sufficient enough to start a standby power source or properly shut down the protected equipment.
FIG. 1 is a simplified schematic of a typical prior art UPS system 100. The basic elements of UPS system 100 are rectifier 102, inverter 104, a DC power source such as battery 106, a controller 108, and a static transfer switch 110. Battery 106 may be coupled through a boost circuit 107 to an input 105 of inverter 104, which is also coupled to an output 103 of rectifier 102. An input 114 of rectifier 102 is coupled through disconnect switch 116 to a primary power source 115 of power, typically an AC feed from a utility. An input 118 of static transfer switch 110 is coupled through disconnect switch 120 to a secondary power source 122 of power, typically an AC feed from a utility, and an output 124 of static transfer switch 110 is coupled to an output 126 of inverter 104. Output 126 of inverter 104 is coupled through a disconnect switch 128 to output 112 of UPS system 100. Output 112 of UPS system 100 is coupled through a manual bypass switch 130 to secondary power source 122. It should be understood that primary power source 115 and secondary power source 122 can be different power sources or the same power source, such as the same utility feed coupled to both disconnect switches 116, 120. Static transfer switch 110 is used to switch load 134 connected to an output 112 of UPS system 100 to secondary power source 122. A fuse is used to protect load 134 and is coupled in series between load 134 and the output 112 of UPS system 100. In this regard, when static transfer switch 110 is closed, the load is connected to secondary power source 122 and when static transfer switch is open, the load is disconnected from secondary power source 122 (unless manual bypass switch 130 has been closed).
Controller 108 controls UPS system 100 including controlling inverter 104 by varying the duty cycle of the switching devices in inverter 104 so that inverter 104 provides a desired output voltage. Controller 108 also controls static transfer switch 110 to cause it to switch between closed and open. Controller 108 can be, be part of, or include: an Application Specific Circuit (ASIC); an electronic circuit; a combinational logic circuit; a field programmable gate array (FPGA); and/or a processor such as a Digital Signal Processor (DSP), microcontroller, or the like. It should be understood that controller 108 may include one or more than one of the foregoing, such as digital controller based on DSPs that control each of the functional blocks of UPS system 100 by generating the proper switching signals to switch the power semiconductors such as IGBTs and thyristors.
Rectifier 102 may be a three phase rectifier having three full rectification legs (and illustratively uses power switching devices such as IGBTs), one for each phase, and inverter 104 may be a three phase inverter having three inverter legs, one for each phase. Inverter 104 also illustratively uses power switching devices such as IGBTs. Rectifier 102 and inverter 104 are configured in a double conversion path with UPS system 100 thus being a double conversion UPS system.
Static transfer switch 110 is typically implemented with power semiconductor switching devices. One type of power semiconductor switching device used in implementing static transfer switches is the thyristor since it is a very robust device, is relatively inexpensive, and has low losses. Typically, a static transfer switch implemented with thyristors has a pair of reverse connected thyristors 132 for each phase. That is, if UPS system 100 is a three phase system, static transfer switch 110 would have three pairs of reverse connected thyristors 132, one for each phase. It should be understood that each thyristor 132 may include a plurality of parallel connected thyristors 132 to provide the requisite power handling capability.